Abstract: Geographic barriers and geological historical events may play pivotal roles in driving allopatric divergence among closely related species. Here, we investigate the genomic divergence patterns and ecological niche separation of the Willow Tit Poecile montanus and the Marsh Tit P. palustris species groups in China, and their ecological niche separation across East Asia. Through comprehensive genomic sequencing, population genomic analysis, and integration of public occurrence data, we unveil striking parallels in the geographic divergence patterns between these two species groups. Notably, both species exhibit multiple divergent lineages in China, with similar spatial distributions of gene flow barriers. Furthermore, our analysis reveals unique evolutionary histories in the southwestern clades of both species groups, highlighting the intricate interplay between historical distribution dynamics, ecological preferences, and genetic divergence. Our study significantly enhances our understanding of the processes underlying the diversification of closely related widespread species within the framework of shared geographical constraints, and stresses the need for a taxonomic revision.

Key words: Marsh Tit, Willow Tit, biogeography, parallel divergence, East Asia